FIG. 1 illustrates a prior art memory system known informally as RamLink, which was proposed as a standard by the Institute of Electrical and Electronics Engineers (IEEE). The standard was designated as IEEE Std 1596.4-1996 and is known formally as IEEE Standard for High-Bandwidth Memory Interface Based on Scalable Coherent Interface (SCI) Signaling Technology (RamLink). The system of FIG. 1 includes a memory controller 10 and one or more memory modules 12. The memory controller 10 is typically either built into a processor or fabricated on a companion chipset for a processor. Each memory module 12 has a slave interface 14 that has one link input and one link output. The components are arranged in a RamLink signaling topology known as RingLink with unidirectional links 16 between components. A control interface 18 on each module interfaces the slave interface 14 with memory devices 20. In the system shown in FIG. 1, another RamLink signaling topology known as SyncLink is used between the slave interfaces and memory devices.
The purpose of the RamLink system is to provide a processor with high-speed access to the memory devices. Data is transferred between the memory controller and modules in packets that circulate along the RingLink. The controller is responsible for generating all request packets and scheduling the return of slave response packets.
A write transaction is initiated when the controller sends a request packet including command, address, time, and data to a particular module. The packet is passed from module to module until it reaches the intended slave, which then passes the data to one of the memory devices for storage. The slave then sends a response packet, which is passed from module to module until it reaches the controller to confirm that the write transaction was completed.
A read transaction is initiated when the controller sends a request packet including command, address, and time to a module. The slave on that module retrieves the requested data from one of the memory devices and returns it to the controller in a response packet, which is again passed from module to module until it reaches the controller.
FIG. 2 illustrates a prior art RamLink slave interface circuit. In the circuit of FIG. 2, source-synchronous strobing is used to clock the incoming data signals. That is, a strobe signal, which accompanies the incoming data signals, is used to sample the incoming data. The circuit of FIG. 2 uses a phase-locked loop (PLL) to generate a stable local clock signal from a reference clock signal that is distributed to other slave interface circuits. The local clock signal is used to reclock the outgoing data signal so as to avoid cumulative jitter as the data is passed along downstream.